Mass producing holograms using embossing tools is known in the prior art. Many charge cards, such as VISA.RTM., MasterCard.RTM., and the like, include an embossed hologram on the card. Embossed holograms are produced in the prior art by impressing a translucent film with an embossing die having a hologram relief pattern.
A hologram embossing die is produced in the prior art using a series of steps. First, a photoresist coated plate is exposed to a holographic interference pattern. The photoresist is then developed to form a surface-relief pattern in the photoresist corresponding to the exposure pattern. A thin layer of nickel is electroformed onto the relief surface of the photoresist. The nickel electroform is then peeled from the photoresist and clamped to a substrate, either by being wrapped around a cylinder or being clamped to a flat block, thus creating the die. A transparent film is then stamped with the metal die, impressing the hologram relief pattern into the film and creating the hologram. (For purposes of this specification, the term hologram is used in the broad sense to include translucent films viewed through laser light, diffraction patterns, "2D3D" holograms, and the other patterns in the broad category of holograms and diffractive patterns.)
The production of embossed holographic pictures using techniques of the prior art has numerous disadvantages. Presently, nickel is the preferred metal for a hologram die. Only material that is softer than the metal of the die can be imprinted by the die. For example, nickel is able to imprint polyester films, some types of plastic, or the like. However, nickel deforms after continued use so that after a few thousand feet of embossing it is no longer usable. A further disadvantage is that when the die is wrapped around a cylinder, a seam runs parallel to the axis of the cylinder. A hologram pattern cannot be printed on or near such a seam. While it is of course possible to electroform other metals than nickel, the problem remains that a solid cylindrical roller cannot be made by a deposition process such as electroplating. Also, nickel is not a suitable material for high-impact processes such as coining for a number of reasons. The master die, which is generally made of nickel, must be firmly clamped to a die block. Because the nickel is relatively soft, it is deformed when clamped to the substrate. In addition, the nickel is not harder than other metals that one may wish to stamp.
The use of chemical etching to transfer photoresist patterns into embossing rollers and dies is known in the prior art. U.S. Pat. No. 3,944,420, to Gale et al., describes a technique for producing an aluminum die for hologram embossing. However, the Gale et al. process is severely limited by the isotropy of this chemical etching process so that the fidelity of the etched pattern degrades rapidly with increasing etch depth. Further, aluminum cannot withstand for long the extremely high pressures present in hologram embossing presses and thus is not suitable for use as a die. Consequently, truly durable metal dies for hologram embossing have not been produced by chemical etching.
Ion milling, reactive-ion etching, and related processes have been practiced in the semiconductor industry. Various ion-milling techniques are used to selectively produce relief patterns in silicon or other semiconductor substrates, as is well known in the microelectronics art. Ion milling has been used to alter the shape of surface-relief holograms recorded in photoresist by taking advantage of the dependence of etch rate on the angle of incidence. For example, sinusoidal relief holograms have been converted to holograms having a sawtooth groove profile by ion milling at appropriate angles to the surface. Reactive-ion etching has been used to etch an interferometrically recorded diffraction grating into the surface of an optical fiber to produce an optical element in the fiber, as described in Laser Focus/Electro-Optics, October 1986, page 74.